There have been a number of techniques which have been developed over the years which have been used to fight fires. Dirt and water have been used for thousands of years as a means to smother fires. More recently, water dispensing systems, including fixed, mobile and portable systems have been developed to facilitate the extinguishing and prevention of fires.
Water alone, though effective, has significant drawbacks, which include weight, availability, resulting damage to property, and failure to adhere to vertical structures. Any reasonable amount of water, which weigh about 8 pounds per gallon, is extremely heavy and either has to be stored on site, made accessible via plumbing systems whose pumps must stay operational during a fire, or transported to the site. Each of these solutions has significant drawbacks.
As a means to address these shortcomings, systems have been developed which use a foaming agent in conjunction with water to create a foam which can be sprayed directly on the material, whether burning or not.
Foams designed to protect material from burning are called pre-suppression foams and use one type of foaming agent. Foams designed to be used on burning material are called suppressant foams and use a separate type of foaming agent.
Foaming techniques are divided into two general classes. These are aspirated foam systems or compressed air foam systems also known as CAFS.
Aspirated foam systems do not use compressed gases to create the foam. A mixture of foam concentrate and water is pumped through a special aspirating nozzle located at the end of the delivery hose. This special nozzle is designed to draw in atmospheric air and mix it with the foam concentrate-water mix to create foam. Because aspirated foam systems are not powered by compressed gases, they do not project the foam a great distance from the end of the nozzle and are meant primarily for laying firebreaks along the ground and down hillsides. Thus aspirated foam systems are often refered to as low energy systems.
Compressed air foam systems operate by a different method. Compressed gas, usually air, is introduced into the foam concentrate-water mixture at high pressure, usually in the range of 60-100 psi. The air is mixed with the foam concentrate-water mixture either with an active mixing device or a passive mixing device which can be as simple as a long length of fire hose. Once the foam, which is now under high pressure, exits the delivery hose, it can travel for quite a distance, usually as much as 70 to 100 feet. Thus compressed air foam systems are referred to as high energy systems. This feature enables the user to direct the foam to hard-to-reach distant locations and keeps the user at a safe distance from the fire.
As discussed above, both the aspirated and CAFS create the foam from a foam concentrate-water mixture. There are several ways in which the mixture can be made.
One method is to simply mix the two components together in one tank and then draw that mixture into the system. This is an acceptable method in some situations, but some foam concentrates have a limited useful life after the foam concentrate has been mixed with water.
Another method is to use an eductor proportioning system which can be located on the inlet side or outlet side of the pump. In either case, the water that is either being drawn into the pump or delivered out of the pump is directed through a venturi-type device which creates a vacuum on an auxiliary inlet port. This auxiliary inlet port is connected to a tank of the foam concentrate. The actual ratio of the two components depends on the such factors as the size of the various inlet and outlet openings, flow rate of the water, and the size of the interior channels of the eductor.
The compressed gas is generally provided by one of two means. The first is a standard high pressure cylinder or gas tank. The second is a mechanically driven air compressor. The compressor can be driven by an electric motor or a gas engine. Use of such devices can be problematic for several reasons. In fire situations, the electrical supply may not be operating due to downed power lines. Likewise, gas engines may not run because the local environment can become oxygen depleted because of the fire. In addition, the fuel stored in the tanks of the gasoline engines can go bad. Gasoline is not a pure component, but rather a mixture of components of varying volatilities in order to provide good running conditions. As gas ages, the lower molecular weight components evaporate. Because these tend to be the more volatile components, older gas doesn't vaporize as well and an engine using the older gas will be harder to start. Thus gasoline driven compressors require considerable maintenance and upkeep in order to be a reliable source of compressed gas.
Another solution is to provide a bank of compressed gas cylinders which will supply the compressed gas for the generation of the foam. The pump can also be designed to be driven by the compressed gas, so that it too is independent of the local electrical supply and/or internal combustion engines.